When a fluid flows around a structure, vortexes whose frequency and even whose appearance are governed by the characteristics of the fluid (velocity, viscosity, density) and of the structure (diameter) may appear. These vortexes excite the structure which starts vibrating and can therefore break due to fatigue failure. The flow of the fluid reaching a cylinder creates vortexes in the fluid situated behind the cylinder, in relation to the direction of flow. These vortexes push the cylinder transversely and cause the structure to move periodically. A <<locking-on>> phenomenon appears then: the natural mode of the structure, whose frequency is close to the <<vortex release>> frequency, influences this <<release>> frequency which tends to become equal to the frequency of the natural mode. The cylinder is then excited at one of its natural frequencies. The response of the cylinder under vibration will thus be dominated by the form of the natural mode.
This type of phenomenon, called VIV (Vortex-Induced Vibration), is notably observed in marine structures comprising petroleum effluent production or drilling risers. In the case of a riser, this phenomenon has to be taken into account for calculating the lifetime, because it can induce considerable structure damage, notably under deep sea conditions, in particular for catenary type risers (SCR).